The field of the invention is solar water heaters and the invention more particularly relates to water heaters of the type utilizing an opaque absorber sheet or member which is placed in a heat transfer relationship with water.
Solar water heaters have been the subject of intense interest in recent years and have been the subject of serious research since at least 1938 when a research team at the Massachussets Institute of Technology built the first in a series of four solar homes. The initial approach at that time was to take a black metal plate, cover it with glass and permit the sun's light to heat the plate. Metal pipes are connected in a heat transfer relationship with the black metal plate and water is pumped through the pipes. Typically the plate is insulated on its back side to prevent conduction of the heat away from the water. The heat which is absorbed by the plate is conducted into the pipe and then into the water.
The majority of solar heaters presently in commercial use utilize copper pipe affixed in heat transfer relationship with a blackened metal plate. Water typically under usual city water pressure is passed through the pipe and into a hot water tank or other reservoir. Another widely used type of heater is a thin film type solar water heater. One such thin film solar heater is disclosed in Yellott U.S. Pat. No. 3,146,774 and utilizes water at essentially atmospheric pressure which is allowed to flow downwardly in a thin film behind a heat absorber collector such as a pliable plastic film. Other flexible elastomeric or plastic water heaters have been utilized such as that disclosed in Andrassy U.S. Pat. No. 3,002,781 and in Okuda U.S. Pat. No. 3,029,806.
Solar water heaters now in commercial use typically store heat in a hot water storage tank. The water in the storage tank may be heated either directly as it comes from the panels or indirectly transferred by a heat exchanger from a solar transfer fluid. This fluid may be used in a closed loop including the solar panel, a pump and a heat exchanger which transfers heat from the fluid to the water in the storage tank.
The solar water heaters in use at the present time all suffer from various shortcomings and have not, at the present writing, found wide spread use in the United States. Solar water heaters made from plastic materials such as polyvinyl chloride have limited life because of the tendancy of plastics to deteriorate in ultra violet light. While various ultra violet light absorbers can be used to improve aging properties, plastic solar heaters still must be replaced frequently. Furthermore if the plastic is exposed to the outside elements it may be damaged by tree limbs, animals, children or the like. Still further, plastic materials tend to be highly insulative and the ability of the solar heat to be conducted through the film is severly limited.
Metal, such as copper and aluminum, eliminate the problem of the poor heat transfer or insulative properties of plastic film solar heaters but in turn have other disadvantages. Aluminum, for instance, is quite corrosive when exposed to typical city water. Aluminum is even more corrosive when exposed to swimming pool water in that the pH of the water often varies widely by the failure to add acid or the addition of too much acid and acidity greatly accelerates the rate of corrosion of aluminum and many other metals. Any connection of aluminum pipe to a dissimilar metal likewise accelerates corrosion by the process known as galvanic action. Thus, at least at the present, copper pipe, because of its relatively high corrosion resistance and excellent heat transfer properties has been the material of most wide-spread acceptance. The cost of copper, however, is relatively high and it is a metal which may become scarce in the future and attempts to conserve copper are desirable.
The rate of heat transfer per square foot of area exposed to the sun is another important consideration of solar water heaters. If this rate is not high the number of square feet necessary to heat the ordinary home becomes impractically large. Typically it would be desirable to provide sufficient heat flow so that the average house could be heated by covering only a portion of the roof of the house. Many of the present commercial water heaters have relatively low heat transfer input per square foot of surface area.